In accordance with recent noticeable developments in portable electronic equipments, communication equipments and the like, the development of secondary batteries with high energy density is strongly demanded from the viewpoint of economic efficiency and reduction in size and weight of the equipment. Currently available secondary batteries with high energy density include a nickel-cadmium battery, a nickel-hydrogen battery, a lithium-ion secondary battery, a polymer battery and the like. Among these batteries, the demand for the lithium-ion secondary battery is strongly growing in the power source market due to its dramatically enhanced life and capacity, compared with the nickel cadmium battery or nickel-hydrogen battery.
FIG. 1 is a view indicating a configuration example of a coin-shaped lithium-ion secondary battery. The lithium-ion secondary battery includes, as shown in FIG. 1, a positive electrode 1, a negative electrode 2, a separator 3 impregnated with electrolyte, and a gasket 4 intended for maintaining the electric insulation between the positive electrode 1 and the negative electrode 2 and sealing what are contained inside the battery. When charging or discharging is performed, lithium ions move between the positive electrode 1 and the negative electrode 2 through the electrolyte in the separator 3.
The positive electrode 1 includes a counter electrode case 1a, a counter electrode current collector 1b, a counter electrode 1c, and lithium cobaltate (LiCoO3) or manganese spinel (LiMn2O4) is mainly used for the counter electrode 1c. The negative electrode 2 includes a working electrode case 2a, a working electrode current collector 2b, and a working electrode 2c, and a negative electrode material used for the working electrode 2c is generally composed of an active material capable of occluding and releasing lithium ions (negative electrode active material), a conductivity agent, and a binder.
As the negative electrode active material for a lithium-ion secondary battery, a composite oxide of lithium and boron, a composite oxide of lithium and transition metal (V, Fe, Cr, Mo, Ni, etc.), a compound containing Si, Ge or Sn, nitrogen (N), and oxygen (O), Si particle whose surface is coated with a carbon layer by means of chemical vapor deposition, and the like have been proposed in the past.
However, each of these negative electrode active materials noticeably deteriorates due to generation of dendrite or a passivated compound on the electrode according to repeated charging and discharging, or shows an increase in the expansion or contraction at the time of occlusion or release of lithium ions, although it can improve the charge and discharge capacities to enhance the energy density. Therefore, lithium-ion secondary batteries using these negative electrode active materials do not have sufficient maintainability of discharge capacity in repeated charging and discharging (hereinafter referred to as “cycle characteristic”). Further, the ratio of discharge capacity to charge capacity just after production (discharge capacity/charge capacity, hereinafter referred to as “initial efficiency”) is also insufficient.
On the other hand, it has been attempted to use a silicon oxide represented by SiOx (0<x≦2), such as SiO, as the negative electrode active material. The silicon oxide can be a negative electrode active material having much increased valid charge and discharge capacities since it is low (less noble) in electrode potential to lithium, and can reversibly occlude and release lithium ions without deterioration such as collapse of crystal structure or generation of irreversible substances attributable to the occlusion and release of lithium ions during charging and discharging. Therefore, the silicon oxide can be expected, by using it as the negative electrode active material, to provide a secondary battery high in voltage and energy density and also excellent in charge-discharge characteristic and cycle characteristic.
As efforts about the above-mentioned negative electrode active material, for example, a nonaqueous electrolyte secondary battery using a silicon oxide, which allows occlusion and release of lithium ions, as the negative electrode active material is proposed in Patent Literature 1. This proposed silicon oxide contains lithium in its crystal structure or amorphous structure, and constitutes composite oxides with lithium and silicon so that lithium ions can be occluded and released by electrochemical reaction in a nonaqueous electrolyte.
In the secondary battery proposed in Patent Literature 1, a high-capacity negative electrode active material can be obtained. However, according to the present inventors' investigations, there is still room for further improvements toward the practical use since the irreversible capacity at the time of initial charging and discharging is significantly notable (namely, the initial efficiency is not sufficient), and the cycle characteristic does not reach a practical level sufficiently